The present invention relates to adjustment of frequency standards and more particularly to a novel method and apparatus for adjusting local frequency sources to a precision not heretofore attainable through the use of network subcarrier sources and offset information regularly published by the National Bureau of Standards (NBS).
A large number of applications in fields such as television transmission, space communications, radar, satellite tracking, as well as other scientific and industrial applications typically require frequency sources having a high degree of stability and precision. Many applications require both short term and long term stability and the presence of these capabilities in a single frequency source is not easily obtainable. Short term stability is typically considered as that interval of time required to make a measurement, which interval is typically in the range from a fraction of a second to a few seconds.
The major television networks of the United States have adopted atomic oscillators for generating color reference signals, which oscillators are typically of the rubidum-vapor or rubidum gas-cell standards which are utilized to produce the necessary color signal at a frequency of 3.58 . . . MHz, also typically referred to as the color subcarrier signal. Such frequency sources are characterized as "secondary" sources, the primary sources, for example, being the cesium type atomic clocks regularly maintained by the National Bureau of Standards, such clocks being categorized as "primary" standards.
The rubidium type frequency standards, while having excellent short term stability, are nevertheless subjected to drift and the output frequency of the standard is regularly monitored by NBS for purposes of comparing the network rubidium frequency standards against the rate of the NBS atomic time scale. The drift results in frequency offset values are periodically published, whereby the offset values represent the frequency offset or difference between the atomic time scale or U.S.F.S. (United States Frequency Standard) and the network rubidium standards to permit adjustment and thereby correct for such drift.
It is thus possible to utilize a standard color receiver as a high precision frequency reference.
The rubidium frequency standards were adopted by the major television networks and these standards were originally referenced to the U.T.C. offset of 300 .times. 10.sup.-.sup.10 with respect to the defined atomic frequency standards. The oscillators employed by the networks have nominal 5 MHz output frequencies, with an approximate -300 .times. 10.sup.-.sup.10 offset and are modified by the use of frequency synthesizers to generate the subcarrier signal of 3.58 MHz (approximately), the aforesaid subcarrier frequency being synthesized by taking 63/88 .times. 5 MHz.
This frequency standard, together with the offsets regularly published by NBS is typically utilized as a calibrating signal for very accurately setting local oscillators.
The standard technique for calibrating a local 5 MHz oscillator is to couple the output of the local oscillator through a frequency synthesizer to generate the subcarrier frequency signal. The color receiver which locally generates the 3.58 MHz signal is then typically compared in either a phase comparator in the form of a phase meter or through the employment of a chart recorder. For example, if the phase meter provides a zero reading (i.e., a zero pointer deflection) or at a constant non-zero reading this indicates that the two signals are operating at the same frequency. Although this represents the ideal situation, as a practical matter, the phase meter, as exemplified by the chart recorder, does not plot a straight line. In fact, the plot deviates quite significantly from a straight line as a result of instabilities in the propagation path due to both small and large phase jumps, changes in network path length and changeovers from network transmission to local station transmission.
Ideally, comparisons and calibrations should be performed during those times in which the networks are presenting "live" programming which presently is typically of the order of 14 hours per day. The phase recording measurement interval comparing the color subcarrier signal of the color receiver with the local oscillator, must be maintained for a period of the order of 15 minutes in order to obtain adequate resolution so that the error is of the order of one part in 10.sup.10. For measurement times shorter than 15 minutes, the resolution is reduced substantially in proportion to the reduction in measurement time. Other techniques which have been developed such as, for example, the TV color bar comparator system and the frequency measurement computer (both developed by the National Bureau of Standards) respectively provide calibration accuracy to the extent that the errors are one part in 10.sup.10 and one part in 10.sup.11 over measurement times of the order of minutes.